Optical systems with compact back focal lengths

ABSTRACT

Optical systems, such as 2-D and 3-D projection systems, may be configured to have a compact back focal length to allow for more compact projection lenses, lower throw ratios, improved contrast, or any combination thereof. In an embodiment, an optical system may include a relay element configured to form an intermediate image having a focal point proximate to a projection lens.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.13/550,213, entitled “Optical systems with compact back focal lengths”,filed Jul. 16, 2012, which is a non-provisional conversion of, and thusclaims priority to, U.S. Provisional Patent Application No. 61/507,959,entitled “Compact polarization converting stereoscopic projection,”filed Jul. 14, 2011, and U.S. Provisional Patent Application No.61/508,428, entitled “Wide throw ratio polarization convertingstereoscopic projection system,” filed Jul. 15, 2011, and is acontinuation-in-part of, and thus claims priority to, U.S. patentapplication Ser. No. 12/118,640, entitled “Polarization ConversionSystem and Method for Stereoscopic Projection,” filed May 9, 2008, allof which are incorporated herein by reference in their entirety.Pursuant to 37 CFR 1.7(b), this application is hereby filed on Monday,Jul. 16, 2012, which is the next succeeding business day following theone year anniversary of the filing of Prov. Pat. App. Nos. 61/507,959and 61/508,428.

TECHNICAL FIELD

The present disclosure generally relates to optical systems, and morespecifically, to two dimensional and three dimensional projectiontechnologies and components.

BACKGROUND

Projection technologies may include functionality to deploy, view,project and/or display three dimensional (“3D”) content. Active andpassive polarization converting stereoscopic projection systems havebeen disclosed in commonly-owned U.S. Pat. Nos. 7,905,602 & 7,959,296and U.S. patent application Ser. Nos. 12/118,640 and 13/034,643, all ofwhich are hereby incorporated by reference in their entirety.

BRIEF SUMMARY

According to an exemplary embodiment, an optical system may comprise animaging source operable to output light, a relay element operable toreceive the light from the imaging source, and a projection lens. Therelay element may be configured to form an intermediate image having afocal point proximate to an entry surface of the projection lens, thefocal point of the intermediate image and the entry surface of theprojection lens defining a back focal length therebetween. Theprojection lens may be operable to project the intermediate image.

According to another exemplary embodiment, an optical system maycomprise an imaging source operable to output light, a relay elementoperable to receive the light from the imaging source, a polarizing beamsplitter assembly, and first and second projection lenses disposedproximate to first and second exit ports of the polarizing beam splitterassembly, respectively. The relay element may be configured to form afirst intermediate image having a focal point between the first exitport of the polarizing beam splitter assembly and the first projectionlens, the focal point of the first intermediate image and an entrysurface of the first projection lens defining a first back focal lengththerebetween. The relay element may be configured to form a secondintermediate image having a focal point between the second exit port ofthe polarizing beam splitter assembly and the second projection lens,the focal point of the second intermediate image and an entry surface ofthe second projection lens defining a second back focal lengththerebetween. The first and second projection lenses may be operable toproject the first and second intermediate images, respectively.

According to another exemplary embodiment, an optical system maycomprise a an imaging source operable to output light, a relay elementoperable to receive the light from the imaging source, a projectionlens, and a polarization conversion system. The relay element may beconfigured to form an intermediate image having a focal point proximateto an entry surface of the projection lens, the focal point of theintermediate image and entry surface of the projection lens defining aback focal length therebetween. The projection lens may be operable toproject the intermediate image through the polarization conversionsystem, which may be operable to convert the light comprising theintermediate image to a single polarization state and output convertedlight along first and second optical paths.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments are illustrated by way of example in the accompanyingfigures, in which like reference numbers indicate similar parts, and inwhich:

FIG. 1A is a schematic diagram illustrating an embodiment of an opticalsystem, in accordance with the present disclosure;

FIG. 1B is a schematic diagram illustrating an embodiment of adual-projector optical system, in accordance with the presentdisclosure;

FIG. 2A is a schematic diagram illustrating an embodiment of an opticalsystem having a polarization beam splitter assembly, in accordance withthe present disclosure;

FIG. 2B is a schematic diagram illustrating an embodiment of adual-projector, optical system having a polarization beam splitterassembly, in accordance with the present disclosure;

FIG. 3A is a schematic diagram illustrating another embodiment of anoptical system having a polarization beam splitter assembly, inaccordance with the present disclosure; and

FIG. 3B is a schematic diagram illustrating another embodiment of adual-projector, optical system having a polarization beam splitterassembly, in accordance with the present disclosure.

DETAILED DESCRIPTION

An increased demand for deploying, viewing, projecting and/or displayingthree dimensional (“3D”) content may drive need for enhanced performanceof projection technology and lower cost. Size of the projection lens andcertain optical components, such as the polarization conversion system(PCS) of a projection system, may affect both performance and cost.Large projection lenses and PCS may limit contrast performance and throwratio. Throw ratio may be defined as a projector to screen distancedivided by the screen width and may be limited resulting from practicaloptical component size limitations. Cost of the projection system mayincrease due to the resulting costs of the larger components.

The size of the polarization converting system (PCS) may be determinedby at least the appropriate throw ratio, the projection lens exit pupillocation, and the size of the projection lens exit pupil. The exit pupilsize may be determined by the projection lens f-number (or numericalaperture) and throw ratio via the optical invariant. Factors that mayaffect the size of a projection lens and certain optical components of aprojection system includes, but not limited to, the appropriate throwratio, f-number, and back focal length (BFL). The throw ratio may bedetermined by the theater geometry. The f-number may be determined bythe panel and illumination optics.

The BFL may be determined by the system architecture. Projectorsutilizing spatial light modulators, for example DLP micro-mirror panels,may employ planar glass optics between the panel and projection lens forvarious reasons, including illumination and color management. Projectionlenses for these projectors may have long BFLs to allow space for theillumination and color management optics between the panel and lens.

But a long back focal length (BFL) through illumination and colormanagement optics in a PCS projection system may increase the size ofprojection lens and other optical components. A large BFL configurationmay compel the lens to have a larger envelope than a lens with a smallerBFL configuration. The larger envelope of the large BFL lens, in turn,may position the exit pupil of the lens further away from the frontsurface vertex of the lens. In this example, the illumination footprintof light emerging from the lens, may be large and may affect the size ofthe PCS components that follow.

Before proceeding to the disclosed embodiments in detail, it should beunderstood that the disclosure is not limited in its application orcreation to the details of the particular arrangements shown, becausethe disclosure is capable of other embodiments. Moreover, aspects of thedisclosure may be set forth in different combinations and arrangementsto define embodiments unique in their own right. Also, the terminologyused herein is for the purpose of description and not of limitation.

An exemplary embodiment of the present disclosure may include the use ofa relay element, such as one or more relay lens, to form an intermediateimage having a focal point proximate to a projection lens, therebyallowing for a relatively short BFL. An exemplary embodiment of thepresent disclosure may include a relay element, at least one short BFLprojection lens, and a polarizing beam-splitter (PBS) assembly. In suchan embodiment, the size of the projection lens and PCS may be reduced,and the uniformity and overall value of the system contrast may beincreased.

FIG. 1 is a schematic diagram illustrating one embodiment of an opticalsystem 100. The optical system 100 may include an imaging source 101operable to output light and a relay element 103 operable to receive thelight from the imaging source 101 either directly or indirectly. Theimaging source 101 may include an illumination source, such as laserillumination light, a lamp source, or a source comprising light emittingdiodes. In an embodiment, the image source 101 may comprise any imagingpanel known in the art, including, for example a spatial modulator or areflective micromirror device. In an embodiment, an optical element 102may be disposed between the imaging source 101 and the relay element 103for managing the illumination or color of light to or from the imagingsource 101. The optical system 100 may further include a projection lens105 operable to project an intermediate image 104. As shown in FIG. 1,the relay element 103 may image the image source 101 through theillumination and color management optics 102 and may create theintermediate real image 104 of the imaging source 101. In an exemplaryembodiment, the relay element 103 is configured to form the intermediateimage 104 such that its focal point is proximate to an entry surface 107of the projection lens 105. The focal point of the intermediate image104 and entry surface 107 of the projection lens 105 may define a backfocal length 108 therebetween.

The projection lens 105 may be located after the intermediate image 104and may project the light or image toward a screen (not shown). Theprojection lens 105 may be various types of lenses, such as, but notlimited to, a zoom lens, a fixed focal length lens, etc. In anembodiment, an active switching or passive polarization component PCS106 may be located after the projection lens 105. By utilizing the relayelement 103 and a projection lens 105 that is relatively short, theprojection lens entrance pupil 111 may be moved closer to the entranceport 112 of the PCS 106. This configuration may allow for a smallerillumination footprint as the light passes through the two optical pathsof the PCS 106. The PCS 106 therefore may service wider throw ratios, oralternatively, the PCS 106 may be reduced in size for a given throwratio.

The relay element 103 may include any relay lens known in the art,including the relay lens described in U.S. Pat. No. 7,317,578, which ishereby incorporated by reference in its entirety. The projection lens105 may include any projection lens known in the art, including theprojection lenses described in J. Brian Caldwell and Ellis I. Betensky,Compact, wide range, telecentric zoom lens for DMD projectors, IODCTechnical Digest, p. 78 (1998), which is hereby incorporated byreference in its entirety. In an embodiment, the relay element 103 andprojection lens 105 may be optimized independently for aberrationcontrol without regard to the other lens' performance. In an embodiment,the two lenses may be designed such that opposing aberrations in the twolenses compensate or substantially null the overall aberrations.Additionally, the relay element 103 and projection lens 105 may bedesigned for higher f-number and higher transmission when lasers areused as the illumination source in the imaging source 101. In anembodiment, the PCS 106 may include anti-reflection elements, mirrors,or polarizing beam-splitter coatings optimized for performance withnarrowband laser illumination.

In one embodiment, the optical system 100 may be configured forprojecting stereoscopic imagery. The relay element 103 may have a longback focal length for imaging the imaging source 101 throughillumination and color management optics 102, and the projection lens105 may have a short back focal length for reducing the distance betweenthe exit pupil and exit surface vertex of the projection lens 105. Thepolarization conversion system 106 may convert randomly or partiallypolarized light to a single polarization state in two separate opticalpaths for overlay on a screen (not shown).

The optical system 100 may be operated with actively modulated PCSs orpassive component PCSs. The PCS 106 may be configured as described inthe commonly-owned U.S. patent application Ser. No. 12/118,640, which ishereby incorporated by reference. As shown in FIG. 1, the PCS 106 mayinclude a polarizing beam splitter 120 operable to split light receivedfrom the projection lens 105 into two paths. The PCS 106 may alsoinclude a reflector 122 to direct light along a first path towards thesame direction as the second light path. The PCS 106 may include lenses130 and 132 configured to substantially match the projected imagemagnification between the first and second optical paths. In anembodiment, the PCS 106 may further include polarization elements 126and 128 disposed in the first and second light paths, respectively, andthey may each comprise a modulator (not shown) operable to switchablymodulate light passing therethrough and time-sequentially output lightof substantially orthogonal polarization states. The polarizationelement 128 may further include polarizers (not shown) and apolarization rotator (not shown) optically preceding the modulator ofthe polarization element 128. The rotator of the polarization element128 may be operable to rotate the polarization state in the second pathto approximately and substantially match the polarization state in thefirst path. In an embodiment, the polarization modulators of thepolarization elements 126 and 128 are operable to sequentially outputlight of first and second orthogonal states towards a screen (notshown), thereby providing stereoscopic images.

In an embodiment, the modulators of the polarization elements 126 and128 may be replaced by one single polarization modulator (not shown)operable to modulate both the first and second light paths. In anembodiment, the polarization modulators 128 and 126 may each be anactive ZScreen as disclosed in the commonly-owned U.S. Pat. No.4,792,850, which is hereby incorporated by reference. In an embodiment,the polarization modulators 128 and 126 may each be a polarizationswitch that was disclosed in the commonly-owned U.S. Pat. No. 7,528,906,which is hereby incorporated by reference.

In an embodiment, the modulator of the polarization element 128 may beconfigured to output light of substantially orthogonal circularpolarization states. In such an embodiment, the polarization element 128may also include a quarter wave plate (not shown) optically followingthe modulator of the polarization element 128 to create substantiallyorthogonal linear polarization states from the circularly polarizedlight emerging from the polarization element 128. The linearpolarization states accumulate lower phase upon reflection at mirror122, and may be then converted back to circular polarization states by aquarter-wave retarder 124 of the PCS 106. This allows for a highercontrast system when the polarization element 128 is located at the exitport of the PBS assembly 120. In another embodiment, the polarizationelement 128 may be moved to the location of quarter wave retarder 124,which along with the quarter wave plate of the polarization element 182may be eliminated, as described in the commonly-owned U.S. patentapplication Ser. No. 12/118,640.

FIG. 1B is a schematic diagram illustrating an exemplary dual-projectoroptical system 150. The optical system 150 comprises two projectionsubsystems 190, 195, and each projection subsystem 190, 195 may have thearchitecture similar to that of the optical system 100. Such adual-projector system may allow for at least one of the following: 1) anincrease in brightness for a given screen size; 2) an increase screensize for a given brightness; 3) some combination of 1) and 2); or 4) astitching of multiple images on-screen to create a higher resolutionimage. In an embodiment, PCS's 106 may be replaced by passive polarizersand/or retarders, allowing the projection subsystem 190 to substantiallyproject a first state of polarization, while the second projectionsubsystem 195 projects a second, substantially orthogonal state ofpolarization.

FIG. 2A is a schematic diagram illustrating one embodiment of anexemplary optical system 200. The optical system 200 may include animaging source 201 operable to output light and a relay element 203operable to receive the light from the imaging source 201 eitherdirectly or indirectly. The imaging source 201 may include anillumination source, such as laser illumination light, a lamp source, ora source comprising light emitting diodes. In an embodiment, the imagesource 201 may comprise any imaging panel known in the art, including,for example a spatial modulator or a reflective micromirror device. Inan embodiment, an optical element 202 may be disposed between theimaging source 201 and the relay element 203 for managing theillumination or color of an image in the light from the imaging source201.

The optical system 200 may further include a polarizing beam splitterassembly 204 and first and second projection lenses 206 a, 206 bdisposed proximate to first and second exit ports 209 a, 209 b of thepolarizing beam splitter assembly 204, respectively. As shown in FIG.2A, the relay element 203 may image the image source 201 through theillumination and color management optics 202 and may create theintermediate real images 205 a, 205 b of the imaging source 201. In anexemplary embodiment, the relay element 203 is configured to form thefirst intermediate image 205 a such that its focal point is between thefirst exit port 209 a of the polarizing beam splitter assembly 204 andthe first projection lens 206 a. The focal point of the firstintermediate image 205 a and an entry surface 211 a of the firstprojection lens 206 a may define a back focal length 210 a therebetween.In an exemplary embodiment, the relay element 203 is configured to alsoform the second intermediate image 205 b such that its focal point isbetween the second exit port 209 b of the polarizing beam splitterassembly 204 and the second projection lens 206 b. The focal point ofthe second intermediate image 205 b and an entry surface 211 b of thesecond projection lens 206 b may define a back focal length 210 btherebetween.

The PBS assembly 204 may be operable to receive light from the relayelement 203 and output substantially orthogonally polarized light ontoat least two distinct optical paths. As shown in FIG. 2A, the assembly204 may include reflecting surfaces 212 for redirecting the two opticalpaths such that they emerge approximately parallel to one another. Onepath may contain an odd number of reflecting surfaces opticallyfollowing a polarizing beam splitter 204 a while the other may containan even number of reflecting surfaces optically following the polarizingbeam splitter 204 a.

Referring to FIG. 2A, in an embodiment, the projection lenses 206 a, 206b may be located after the intermediate images 205 a, 205 b and projectthe respective individual images toward a screen (not shown). Theprojection lenses 206 a, 206 b may be any type of suitable lenses suchas, but not limited to, zoom lenses, fixed focal length lenses, etc.Active switching or passive polarization components 207 a, 207 b mayoptically follow the projection lenses 206 a, 206 b, respectively, formodulating or passively altering the polarization state emerging fromeach projection lens 206 a, 206 b. A rotator 208 may be disposed in oneoptical path between one of the exit ports 209 a, 209 b and polarizationcomponent 207 a, 207 b, respectively. The rotator 208 may rotate thepolarization state in a first path to approximately and substantiallymatch the polarization state in a second path. In an embodiment, thepolarization components 207 a, 207 b may each comprise a passivepolarizer or retarder. In another embodiment, polarization components207 a, 207 b may each comprise a polarization modulator operable to beactively switched to output light of orthogonal polarizations. Such anembodiment may be used to provide stereoscopic imaging.

It is to be appreciated that by configuring the relay element 203 tolocate the focal points of the intermediate images 205 a, 205 bproximate to and between the exit ports 209 a, 209 b of the PBS assembly204 and the projection lenses 206 a, 206 b, respectively, a shorter BFLmay be achieved to allow for more compact projection lenses 206 a, 206b, which may result in a more compact and less costly system.Furthermore, the compact projection lens 206 a, 206 b may allow for asmaller illumination footprint as the light passes through thepolarization components 207 a, 207 b, and 208, thus reducing the size ofthese components. Generally, the smaller polarization components may be,the easier to manufacture. And with smaller components, wider projectionangles and lower throw ratios may be realized.

FIG. 2B is a schematic diagram illustrating an exemplary dual-projectoroptical system 250. The optical system 250 comprises two projectionsubsystems 290, 295, and each projection subsystem 290, 295 may have anarchitecture similar to that of the optical system 200. Such adual-projector system may allow for at least one of the following: 1) anincrease in brightness for a given screen size; 2) an increase screensize for a given brightness; 3) some combination of 1) and 2); or 4) astitching of multiple images on-screen to create a higher resolutionimage. In an embodiment, the polarization components 207 a, 207 b mayeach comprise a passive polarizer and/or a retarder, allowing theprojection subsystem 290 to substantially project a first state ofpolarization, while the second projection subsystem 295 projects asecond, substantially orthogonal state of polarization.

FIG. 3A is a schematic diagram illustrating one embodiment of an opticalsystem 300. Similar to optical system 200, the optical system 300 mayinclude an imaging source 301 operable to output light and a relayelement 303 operable to receive the light from the imaging source 301either directly or indirectly. The imaging source 301 may include anillumination source, such as laser illumination light, a lamp source, ora source comprising light emitting diodes. In an embodiment, the imagesource 301 may comprise any imaging panel known in the art, including,for example a spatial modulator or a reflective micromirror device. Inan embodiment, an optical element 302 may be disposed between theimaging source 301 and the relay element 303 for managing theillumination or color of an image in the light from the imaging source301.

The optical system 300 may further include a polarizing beam splitterassembly 304 and first and second projection lenses 306 a, 306 bdisposed proximate to first and second exit ports 309 a, 309 b of thepolarizing beam splitter assembly 304, respectively. As shown in FIG.3A, the relay element 303 may image the image source 301 through theillumination and color management optics 302 and may create theintermediate real images 305 a, 305 b of the imaging source 301. In anexemplary embodiment, the relay element 303 is configured to form thefirst intermediate image 305 a such that its focal point is between thefirst exit port 309 a of the polarizing beam splitter assembly 304 andthe first projection lens 306 a. The focal point of the firstintermediate image 305 a and an entry surface 311 a of the firstprojection lens 306 a may define a back focal length 310 a therebetween.In an exemplary embodiment, the relay element 303 is configured to alsoform the second intermediate image 305 b such that its focal point isbetween the second exit port 309 b of the polarizing beam splitterassembly 304 and the second projection lens 306 b. The focal point ofthe second intermediate image 305 b and an entry surface 311 b of thesecond projection lens 306 b may define a back focal length 310 btherebetween. In an embodiment, the projection lenses 306 a, 306 b maybe located after the intermediate images 305 a, 305 b and project therespective individual images toward a screen (not shown).

As shown in FIG. 3A, a first optical path of the PBS assembly 304 mayinclude one reflection optically following a polarizing beam splitter304 a, and a second path may include no reflections. In an embodiment,the reflection in the first path may be provided by a prism 304 bdisposed on the top of the polarizing beam splitter 304. One advantageof this embodiment may be that less glass is employed in the PBSassembly 304 when compared to the PBS assembly 204 of FIG. 2. In anembodiment, a polarization rotator 308 may be disposed in one path andpolarization components 307 a and 307 b may be disposed in both pathsfollow the projection lenses 306 a, 306 b. In an embodiment, thepolarization components 307 a, 307 b may each comprise a passivepolarizer or retarder. In another embodiment, polarization components307 a, 307 b may each comprise a polarization modulator operable to beactively switched to output light of orthogonal polarizations. Such anembodiment may be used to provide stereoscopic imaging.

The system 300 may include actively modulated PCSs or passive componentPCSs. In both cases, either active or passive, multiple PCSs may be usedon multiple projectors. FIG. 3B is a schematic diagram illustrating anexemplary dual-projector optical system 350. The optical system 350comprises two projection subsystems 390, 395, and each projectionsubsystem 390, 395 may have an architecture similar to that of theoptical system 300. Such a dual-projector system may allow for at leastone of the following: 1) an increase in brightness for a given screensize; 2) an increase screen size for a given brightness; 3) somecombination of 1) and 2); or 4) a stitching of multiple images on-screento create a higher resolution image. In an embodiment, the polarizationcomponents 307 a, 307 b may each comprise a passive polarizer and/or aretarder, allowing the projection subsystem 390 to substantially projecta first state of polarization, while the second projection subsystem 395projects a second, substantially orthogonal state of polarization.

It should be noted that embodiments of the present disclosure may beused in a variety of optical systems and projection systems to allow fora compact back focal length, which allows for more compact projectionlenses, lower throw ratios, improved contrast, or any combinationthereof. The embodiment may include or work with a variety ofprojectors, projection systems, cameras, image capture devices, opticalcomponents, computer systems, processors, self-contained projectorsystems, visual and/or audiovisual systems and electrical and/or opticaldevices. Aspects of the present disclosure may be used with practicallyany apparatus related to optical and electrical devices, opticalsystems, capture systems, presentation systems or any apparatus that maycontain any type of optical system. Accordingly, embodiments of thepresent disclosure may be employed in optical systems, devices used invisual and/or optical presentations, visual peripherals and so on and ina number of computing environments including the Internet, intranets,local area networks, wide area networks and so on.

As may be used herein, the terms “substantially” and “approximately”provide an industry-accepted tolerance for its corresponding term and/orrelativity between items. Such an industry-accepted tolerance rangesfrom less than one percent to ten percent and corresponds to, but is notlimited to, component values, angles, et cetera. Such relativity betweenitems ranges between less than approximately one percent to ten percent.

While various embodiments in accordance with the principles disclosedherein have been described above, it should be understood that they havebeen presented by way of example only, and not limitation. Thus, thebreadth and scope of this disclosure should not be limited by any of theabove-described exemplary embodiments, but should be defined only inaccordance with any claims and their equivalents issuing from thisdisclosure. Furthermore, the above advantages and features are providedin described embodiments, but shall not limit the application of suchissued claims to processes and structures accomplishing any or all ofthe above advantages.

Additionally, the section headings herein are provided for consistencywith the suggestions under 37 CFR 1.77 or otherwise to provideorganizational cues. These headings shall not limit or characterize theembodiment(s) set out in any claims that may issue from this disclosure.Specifically and by way of example, although the headings refer to a“Technical Field,” the claims should not be limited by the languagechosen under this heading to describe the so-called field. Further, adescription of a technology in the “Background” is not to be construedas an admission that certain technology is prior art to anyembodiment(s) in this disclosure. Neither is the “Summary” to beconsidered as a characterization of the embodiment(s) set forth inissued claims. Furthermore, any reference in this disclosure to“invention” in the singular should not be used to argue that there isonly a single point of novelty in this disclosure. Multiple embodimentsmay be set forth according to the limitations of the multiple claimsissuing from this disclosure, and such claims accordingly define theembodiment(s), and their equivalents, that are protected thereby. In allinstances, the scope of such claims shall be considered on their ownmerits in light of this disclosure, but should not be constrained by theheadings set forth herein.

What is claimed is:
 1. An optical system, comprising: a relay elementadapted to receive image light; and a projection lens; wherein the relayelement is configured to form an intermediate image having a focal pointproximate to an entry surface of the projection lens, the focal point ofthe intermediate image and the entry surface of the projection lensdefining a back focal length therebetween; and wherein the projectionlens is adapted to project the intermediate image.
 2. The optical systemof claim 1, wherein the optical system further comprises a polarizationconversion system adapted to convert the light comprising theintermediate image to substantially a single polarization state andoutput converted light along first and second optical paths.
 3. Theoptical system of claim 1, further comprising a polarizing beam splitterassembly and a second projection lens, wherein the projection lens andthe second projection lens are disposed proximate to first and secondexit ports of the polarizing beam splitter assembly, respectively, andwherein the relay element is configured to form a second intermediateimage having a focal point proximate to an entry surface of the secondprojection lens, the focal point of the second intermediate image andthe entry surface of the projection lens defining a back focal lengththerebetween.
 4. The optical system of claim 1, wherein the relayelement is configured to compensate for aberration.
 5. The opticalsystem of claim 1, wherein the projection lens is configured tocompensate for aberration.
 6. The optical system of claim 1, wherein therelay element and the projection lens are configured to cooperate tocompensate an overall aberration.
 7. The optical system of claim 1,wherein the projection lens comprises a zoom lens or a fixed focallength lens.
 8. The optical system of claim 1, wherein the relay elementcomprises a first relay element, and the projection lens comprises afirst projection lens, and wherein the first relay element and the firstprojection lens are comprised in a first projection subsystem, and theoptical system further comprises a second projection subsystemcomprising: a second relay element adapted to image light; and a secondprojection lens; wherein the second relay element is configured to forma second intermediate image having a focal point proximate to an entrysurface of the second projection lens, the focal point of the secondintermediate image and the entry surface of the second projection lensdefining a back focal length therebetween; and wherein the secondprojection lens is adapted to project the second intermediate image. 9.An optical system, comprising: a relay element adapted to receive imagelight; a polarizing beam splitter assembly; and first and secondprojection lenses disposed proximate to first and second exit ports ofthe polarizing beam splitter assembly, respectively; wherein the relayelement is configured to form a first intermediate image having a focalpoint between the first exit port of the polarizing beam splitterassembly and the first projection lens, the focal point of the firstintermediate image and an entry surface of the first projection lensdefining a first back focal length therebetween; wherein the relayelement is configured to form a second intermediate image having a focalpoint between the second exit port of the polarizing beam splitterassembly and the second projection lens, the focal point of the firstintermediate image and an entry surface of the second projection lensdefining a second back focal length therebetween; wherein the first andsecond projection lenses are adapted to project the first and secondintermediate images, respectively.
 10. The optical system of claim 9,wherein the polarizing beam splitter assembly comprises: a polarizingbeam splitter; an odd number of reflectors optically following thepolarizing beam splitter in a first output light path; and an evennumber of reflectors optically following the polarizing beam splitter ina second output light path.
 11. The optical system of claim 9, whereinthe polarizing beam splitter assembly comprises: a polarizing beamsplitter; a single reflector optically following the polarizing beamsplitter in a first output light path; and no reflector opticallyfollowing the polarizing beam splitter in a second output light path.12. The optical system of claim 9, further comprising first and secondpolarization elements optically following the first and secondprojection lens, respectively, and a polarization rotator disposedbetween the first exit port of the polarization beam splitter assemblyand the first polarization element.
 13. The optical system of claim 12,wherein the first and second polarization elements each comprise apassive polarizer or retarder.
 14. The optical system of claim 12,wherein the first and second polarization elements each comprise apolarization modulator adapted to output light of orthogonalpolarizations.
 15. The optical system of claim 9, wherein the relayelement comprises a first relay element, and the polarizing beamsplitter assembly comprises a first polarization beam splitter assembly,and wherein the first relay element, the first and second projectionlenses, and the first polarization beam splitter assembly are comprisedin a first projection subsystem, and the optical system furthercomprises a second projection subsystem comprising: a second relayelement adapted to receive image light; a second polarizing beamsplitter assembly; and third and forth projection lenses disposedproximate to first and second exit ports of the second polarizing beamsplitter assembly, respectively; wherein the second relay element isconfigured to form a third intermediate image having a focal pointbetween the first exit port of the second polarizing beam splitterassembly and the third projection lens, the focal point of the thirdintermediate image and an entry surface of the third projection lensdefining a third back focal length therebetween; wherein the secondrelay element is configured to form a fourth intermediate image having afocal point between the second exit port of the second polarizing beamsplitter assembly and the fourth projection lens, the focal point of thefourth intermediate image and an entry surface of the fourth projectionlens defining a fourth back focal length therebetween; and wherein thethird and fourth projection lenses are adapted to project the third andfourth intermediate images, respectively.
 16. An optical system,comprising: a relay element adapted to receive image light; a projectionlens; and a polarization conversion system; wherein the relay element isconfigured to form an intermediate image having a focal point proximateto an entry surface of the projection lens, the focal point of theintermediate image and entry surface of the projection lens defining aback focal length therebetween; and wherein the projection lens isadapted to project light comprising the intermediate image through thepolarization conversion system, the polarization conversion system beingadapted to convert the light comprising the intermediate image tosubstantially a single polarization state and output converted lightalong first and second optical paths.
 17. The optical system of claim16, wherein the polarization conversion system comprises a polarizingbeam splitter adapted to split light received from the projection lensinto first and second light paths and first and second polarizationelements disposed in the first and second light paths, respectively, andwherein the first and second polarization elements are each adapted toswitchably modulate light passing therethrough and output light ofsubstantially orthogonal polarization states.
 18. The optical system ofclaim 16, wherein the polarization conversion system further comprises areflector to direct light along a first path towards a same direction asthe second light path.
 19. The optical system of claim 16, wherein thefirst and second polarization element are adapted to sequentially outputlight of the substantially orthogonal polarization states, therebyproviding stereoscopic images.
 20. The optical system of claim 16,wherein the relay element is a first relay element, the projection lenscomprises a first projection lens, and the polarization conversionsystem comprises a first polarization conversion system, and wherein thefirst relay element, the first projection lens, and the firstpolarization conversion system are comprised in a first projectionsubsystem, and the optical system further comprises a second projectionsubsystem comprising: a second relay element adapted to receive imagelight; a second projection lens; and a second polarization conversionsystem; wherein the second relay element is configured to form a secondintermediate image having a focal point proximate to an entry surface ofthe second projection lens, the focal point of the second intermediateimage and the entry surface of the second projection lens defining aback focal length therebetween; and wherein the second projection lensis adapted to project light comprising the second intermediate imagethrough the second polarization conversion system, the secondpolarization conversion system being adapted to convert the lightcomprising the second intermediate image to substantially a singlepolarization state and output converted light along first and secondoptical paths; wherein the second polarization conversion systemcomprises a second polarizing beam splitter adapted to split lightreceived from the second projection lens into third and fourth lightpaths and third and fourth polarization elements disposed in the thirdand fourth light paths, respectively, and wherein the third and fourthpolarization elements are each adapted to switchably modulate lightpassing therethrough and sequentially output light of substantiallyorthogonal polarization states, thereby providing stereoscopic images.